The present invention relates to an oil recovery process in which an aqueous alkaline solution containing a water-soluble polymer for reducing the mobility of the solution is injected into a subterranean reservoir to displace oil within the reservoir to a production location. More particularly, the invention relates to such a process in which the aqueous alkaline solution has been substantially freed of dissolved oxygen and is thickened with a partially hydrolyzed polyacrylamide polymer.
With respect to fluid drive oil recovery processes, it is known to use water-soluble anionic polymers as mobility reducing agents. For example, U.S. Pat. No. 3,208,518 suggests using such a solution with an initial pH which reduces the solution viscosity during injection and then rises to increase the solution viscosity within the reservoir. U.S. Pat. No. 3,343,601 suggests that polymer-thickened floor water be deoxygenated by adding a water-soluble hydrosulfite before or after adding the polymer. U.S. Pat. No. 3,581,824 suggests that, within a reservoir, a polymer solution be contacted with an aqueous solution containing divalent cations to agglomerate the polymer, for selectively plugging portions of the reservoir. U.S. Pat. No. 3,676,494 describes sulfur-containing aromatic carboxylic acid amides and indicates that they can stabilize oxygen-sensitive organic materials against oxidation. U.S. Pat. No. 3,801,502 suggests that the viscosity increasing capability of xanthan gum polymers be increased by adding various materials inclusive of water-soluble alcohols.
U.S. Pat. No. 4,317,759 discloses that a combination of a phenolic antioxidant and a mercaptobenzimidazole can stabilize an aqueous polyacrylamide polymer solution. Japanese Pat. No. Sho 49[1974]-27659 relates to stabilizing certain acrylamide polymers for certain uses, such as an aggregation agent, in which the desired effect is higher in the case of higher-molecular-weight polymers (e.g. molecular weights of at least 700,000 or more). The patent is specifically directed to treating polymers in which no more than 50 mole percent of amido groups within a homopolymer may be converted to other functional groups, such as acrylate groups formed by means of chemical reactions such as alkaline hydrolysis. The specified stabilization is said to be accomplished by adding at least one type of 2-mercaptobenzothiazole or its water-soluble salt to the polymer.
In an oil recovery process in which oil is displaced within a subterranean reservoir by injecting a polyacrylamide-polymer-thickened aqueous alkaline solution, relatively unique problems are created by factors such as the solution alkalinity, the reservoir temperature, the duration of the operations, etc. Such problems are discussed in SPE Paper No. 13585, "Chemical Stability Limits of Water Soluble Polymer Used in Oil Recovery Processes" by R. G. Ryles. As indicated in the paper, the commercially available polyacrylamide polymers are typified by initial mole percentage concentrations of acrylate groups in the order of 26 to 30 percent, due to copolymerization or hydrolysis of acrylamide groups, with the polymers having molecular weights in the order of 7 to 14 million. Hydrolysis occurs when water reacts with amide linkages in the polymer. Based on anaerobic stability tests for defining the stability limits of such polyacrylamide polymers in alkaline aqueous solutions, it was found that, even in reservoirs with temperatures as low as 50.degree. C. (122.degree. F.), the mole proportion of the acrylate groups would be greater than 50 percent within less than about two weeks, due to hydrolysis occurring in the reservoir. Although in the presence of a neutral solution with free hardness ions, this hydrolysis may cause the polymer solution to lose viscosity and become less effective, in the presence of an alkaline solution, the hydrolysis provides a beneficial effect by increasing the viscosity of the polymer solution.
The chemical composition of water-soluble polyacrylamide polymers which are effective as water-thickening agents for aqueous alkaline solutions is such that the polymers are susceptible to chemical degradation. Such a degradation, which increases with increasing temperature, reduces the viscosity of a solution containing the polymers. The typical cause of such a degradation is a free-radical reaction. Free-radical reactions are usually those initiated when the polymer solution is mixed with air or oxygen. Such an oxygen-containing mixture tends to form hydroperoxides, and it is the decomposition of the hydroperoxides to peroxide radicals that initiate polymer degradation.